With miniaturization of electronic equipment, a switching power supply has been reduced in size more and more. The switching power supply can be miniaturized by reducing the size of devices used therein and mounting them with high density. However, a temperature increase due to the losses that occur in respective devices cannot be avoided. Therefore, each device must be a low-loss device and must have thermal resistance. Further, a capacitor applied to a switching power supply is used under severe conditions not only in a thermal environment but also in an electric environment. In other words, a high DC bias voltage or a large-amplitude high frequency voltage is applied or a great ripple current flows in many cases. An example of such a capacitor used in a switching power supply is a so-called metalized film capacitor obtained by depositing aluminum or the like on an organic film. A dielectric such as a polyethylene terephthalate film used in this capacitor is excellent in a high frequency property and is stable for high electric field strength. However, the dielectric has a low dielectric constant and a small capacity obtained per unit volume, resulting in a large size inevitably. In addition, the dielectric is difficult to be considered as a low-loss dielectric due to high equivalent series inductance and high equivalent series resistance. Furthermore, since a resin film is used as a base in this capacitor, thermal resistance also has been a matter to be considered.
In order to improve the thermal resistance, it is conceivable to use a multilayer ceramic capacitor formed by laminating dielectric ceramics and electrode metals alternately. The conventional technique of a multilayer ceramic capacitor will be explained roughly.
When considering static capacitance obtained in view of a configuration of a multilayer capacity, the static capacitance increases in proportion to an electrode surface area and the number of laminated layers and in inverse proportion to the thickness of one dielectric layer. The dielectric layer cannot be reduced in thickness excessively when considering its reliability. Therefore, when producing a large-capacity multilayer capacitor, the whole electrode surface area (the product obtained by multiplying an area of one layer by the number of laminated layers) must be increased.
On the other hand, the electrode metal is selected depending on the temperature at which a dielectric is sintered and atmosphere conditions. In other words, when a dielectric must be baked at a temperature of at least 1150.degree. C. in an air atmosphere, palladium or an alloy containing palladium as the main component is selected so that the electrodes are not melted and oxidized. In this case, however, since palladium is expensive, a capacitor with electrodes having a large total area also becomes expensive. When using a dielectric that can be sintered at a temperature of at least 1150.degree. C. in a reducing atmosphere, nickel electrodes can be used, which is favorable in cost. However, materials that are not decreased in dielectric properties, particularly insulation resistance even after being baked in the reducing atmosphere are limited and a high dielectric constant is difficult to obtain. When the firing temperature is below 1150.degree. C., an alloy containing silver as the main component, namely an alloy of silver:palladium=7:3 can be used, which is advantageous in producing a large-capacity multilayer capacitor.
Dielectric ceramic materials used for multilayer ceramic capacitors are roughly divided into two kinds. First materials are paraelectric materials formed of a solid solution containing calcium titanate or strontium titanate as the main component, which are known as materials used for temperature compensation. The paraelectric materials do not have a ferroelectric phase transition point, i.e. a Curie point, even at a low temperature. Second materials are ferroelectric materials containing barium titanate as the main component that are known as materials with a high dielectric constant, or ferroelectric materials called lead-based relaxers containing lead-magnesium niobate or lead-zinc niobate as the main component, which has a Curie point. The ferroelectric materials show the dielectric constant peak at the Curie point. Therefore, when the Curie point is present in the vicinity of room temperature, a higher dielectric constant can be obtained. At a temperature below around the Curie point, dielectric losses (tan .delta.) increase. In view of the above, materials having a Curie point at a temperature somewhat lower than room temperature are selected in high dielectric materials. The first materials are excellent in high frequency property and stable for a DC bias electric field or a large-amplitude high frequency electric field, and exhibit excellent properties even in a severe electrical environment. However, the dielectric constants of the first materials are no more than 300. Therefore, in producing capacitors with a large capacity, for example, at least 1 .mu.F that is required in a power circuit, the capacitors are not only increased in size but also in number of laminated layers, resulting in high costs. Consequently, such capacitors have not been put into practical use. On the other hand, the second materials with high dielectric constants have different dielectric constants depending on their temperature characteristics. However, when using materials having a characteristic of a temperature change rate within.+-.10%, which is stipulated as the B-level characteristic in JIS (Japanese Industrial Standard) C6429, a dielectric constant of 2000 to 3000 is obtained. As materials having a characteristic in which the temperature change is allowed between +30% and -80%, which is stipulated as the F-level characteristic, when using barium titanate-based materials and a lead-based relaxer, high dielectric constants of about 10000 and 15000 to 20000 are obtained, respectively. Thus, the second materials are advantageous in producing capacitors with a large capacity and thus have been used practically.
Materials containing (SrPb)TiO.sub.3 as a main component have been reported as materials used for high voltage capacitors (Hirotaka YAMAMOTO et al., Nihon Ceramics Kyokai Gakujutsu Ronbunshi (Journal of the Ceramic Society of Japan), Vol. 97, No. 6, pages 619-622, 1989, and Publication of Unexamined Japanese Patent Application (Tokkai Sho) No. 60-189107). These reports describe that the materials have a high dielectric constant of 2800 and are not greatly changed in static capacity for a DC bias voltage.
However, when a DC bias voltage is applied to a material with a high dielectric constant, piezoelectricity is induced and thus mechanical vibration is induced by a superimposed high frequency electric field through electromechanical coupling. For instance, when the material is used for an input or output capacitor in a switching power supply and the vibration resonates at the switching frequency, an unwanted ripple current flows, thus increasing loss energy in the capacitor, which contributes to temperature rise. As an experiment, a multilayer capacitor that was formed of dielectrics represented by a formula, Pb.sub.0.94 Sr.sub.0.05 (Mg.sub.1/3 Nb.sub.2/3).sub.0.49 (Zn.sub.1/3 Nb.sub.2/3).sub.0.21 Ti.sub.0.215 (Ni.sub.1/2 W.sub.1/2).sub.0.0425 O.sub.3, that were lead-based relaxers and electrodes made of a silver-palladium alloy was produced, and was used in the above-mentioned circuit. The capacitor had an outer size of 5.times.6.times.1.6 mm, an electrode surface area per layer was 20 mm.sup.2, the number of effective layers was 20, and one dielectric layer had a thickness of 23 .mu.m. In this capacitor, the fluctuation in frequency of the impedance was checked under the application of no DC bias voltage. As shown in FIG. 1(a), LC resonance was found in the vicinity of 1 MHz, but no resonance was found at other frequencies. However, when a DC bias voltage of 40 V was applied to the capacitor, as shown in FIG. 1(b), unwanted resonances were observed in the vicinity of 300 kHz and 2 MHz. It was proved that vibrations in an inplane direction and in a thickness direction were excited due to piezoelectricity induced by the DC bias voltage, which resonated at frequencies determined by the sizes in length and width and the size in thickness of the capacitor, thus causing the unwanted resonances. Even when a DC bias voltage of about 5 V was applied, the above-mentioned unwanted resonances still occurred, although they became weak.
In materials with high dielectric constants, the increase in dielectric loss depending on high frequency electric field strength cannot be avoided. For instance, in the above-mentioned capacitor, a value of tan .delta. was about 0.5% when being measured with a signal of 0.01 V and 1 kHz. However, when being measured with a signal of 10 V and 1 kHz, the value of tan .delta. increased to 1.2%. When a 0.22 .mu.F multilayer capacitor to which commercially available barium titanate dielectrics were applied, which was employed as a capacitor having the B-level characteristic described above, was measured with a signal of 1 kHz, the value of tan .delta. was 0.75% at 0.01 V but increased to 3.5% by the application of a voltage of 15 V. In this capacitor, a dielectric had a thickness of 55 .mu.m. Usually, a switching power supply operates at frequencies of at least several tens to several hundreds kHz. In this case, the increase of tan .delta. depending on the high frequency electric field strength increases the loss in the circuit.
In addition, materials containing (SrPb)TiO.sub.3 as a main component have a high firing temperature of 1220.degree. C. and therefore cannot be used for obtaining an inexpensive multilayer capacitor.